The present invention generally relates to image transmission systems, and more particularly to an image transmission system suitable for use in a video phone, a facsimile machine and the like.
Various video phone systems have been proposed in the past. The video phone was developed mainly in the United States around 1950 to 1970 and experimental results on various systems were announced, but none were reduced to practice. The fundamental problems of the proposed video phone were that the system required a wide band video line exclusively for image transmission. However, the need for setting up the wide band video line exclusively for the video phone made it impractical costwise.
Coming into the 1980's, it has now become possible to reduce into practice a digital data line capable of data transmission at a transmission rate in the order of 56 bps to 64 bps. And, new image transmission systems have been proposed by combining such a digital data line with advanced digital image data compression techniques. But still, such new image transmission systems have not been reduced to practice since the digital data line above described has yet to come into wide use.
There are also image transmission systems which transmit a still picture on a public analog telephone line by using the digital image data compression technique. However, these systems take a long time to transmit the still picture. For example, it takes several tens of seconds to several minutes to transmit one still picture. Accordingly, these systems are not of much use in a personal video phone from the practical point of view.
There is a personal video phone employing a system which transmits one still picture within a few seconds by sacrificing the picture size and picture quality, that is, greatly reducing the quantity of transmitting image data. But this system is restricted to the transmission of a black-and-white picture.
In order to transmit image data related to a color picture, the quantity of the transmitting image data becomes extremely large and it inevitably takes a long time to transmit the image data. As a result, it is extremely difficult to realize an inexpensive personal video phone which can transmit the image data related to the color picture quickly without greatly deteriorating the picture quality.
When transmitting a still picture or a picture having negligibly small movements therein from a video phone, a facsimile machine and the like, a differential pulse code modulation (hereinafter simply referred to as a DPCM) or a delta modulation is usually used for efficiency. The still picture may be a page of a book, a scenery, a portrait and the like.
The DPCM is a predictive coding as is well known, and uses a correlation between picture element data or line data. In other words, the DPCM predicts by use of this correlation a value of a present picture element data from a value of a picture element data which is already encoded, and encodes a difference between the predicted value with an actual value. A difference signal is pulse code modulated into three to four bits.
On the other hand, the delta modulation approximates a signal waveform by a staircase wave having an amplitude which varies by .+-..DELTA., and obtains one step of the staircase wave as a binary code. Hence, the delta modulation essentially quantizes the difference signal in the DPCM into one bit. According to the delta modulation, the quantization step size (width) is constant with respect to a change in the data.
When the DPCM is used for an image data compression system to separate color image data related to a relatively small picture into a luminance signal and color difference signals, independently encode the luminance signal and the color difference signals and transmit the encoded data on the public line, the picture quality of the transmitted image data is satisfactory but the data compression rate is insufficient. As a result, the data transmission cannot be completed within a short time.
The data compression rate is sufficient when the delta modulation is used for the image data compression system, but noise becomes conspicuous with respect to the luminance signal, and it is impossible to obtain a satisfactory picture quality from the transmitted image data.
On the other hand, a delta modulation with adaptive control (hereinafter simply referred to as adaptive delta modulation) can also be used for transmitting a still picture. The adaptive delta modulation uses the fact that the difference between mutually adjacent samples becomes small as the sampling frequency becomes high, and encodes the difference signal into one bit by carrying out the sampling at a high frequency. When encoding the difference signal into one bit, the quantization step size is determined from past transmission pulse train. In other words, when pulses of the same polarity are repeated, the quantization step size is increased so as to follow a large difference between the mutually adjacent samples. On the other hand, the quantization step size is decreased to suppress the quantization noise when pulses of different polarities occur. Hence, according to the adaptive delta modulation, the quantization step size varies with a predetermined rate with respect to a change in the data.
However, because 50% or more picture element data out of the total picture element data constituting a still picture usually have no change in the tone. For this reason, the tone of a reproduced picture becomes unstable when the delta modulation or the adaptive delta modulation is used. To the human eye, it is a large tone change in the still picture that has a large effect visually.